Control valves are typically operated by an actuator, such as a pneumatic actuator, an electric actuator, a hydraulic actuator, etc. The actuator is typically coupled to the control valve and provides the force to move a valve plug to control a fluid flowing through the control valve. For instance, in a pneumatic actuator, increasing or decreasing air pressure moves a diaphragm, which in turn moves an actuator rod that is attached to the center of the diaphragm along the longitudinal axis of an actuator housing. Thus, changes in the air pressure correspond directly to changes in the axial position of the actuator rod.
The actuator rod may be attached to a valve stem that protrudes from the valve body. By mechanically coupling the actuator rod to the valve stem via a valve stem connector, the position of the attached valve plug can be determined by the actuator rod to control the fluid flowing through the valve. Typically, the valve stem connector consists of a single, rigid, connector that includes two threaded cavities for receiving externally threaded ends of the actuator rod and valve stem. However, known valve stem connectors may suffer from certain manufacturing disadvantages and design limitations.
FIGS. 1 and 2 show a cross-sectional view of one example of a typical valve assembly 100 that includes a valve stem connector 102 mechanically fastening an actuator rod 104 to a valve stem 106. The actuator rod 104 is housed in an actuator 108, for example, a pneumatic actuator, and when a supplied air pressure to the actuator 108 changes, the actuator rod 104 moves along a longitudinal axis A-A. The valve stem connector 102 couples the axial motion supplied by the actuator rod 104 through the valve stem 106 to a valve plug 110, which is located in a valve body 120, to allow the valve plug 110 to be positioned relative to a valve seat 112. For example, when the valve plug 110 is positioned away from the valve seat 112, fluid can flow from a valve inlet 114 to a valve outlet 116 as indicated by the arrows shown.
As shown in greater detail in FIG. 2, the illustrated valve stem connector 102 includes two connector portions 118a-b that are fastened by bolts 122a-b to form a rigid connector. The valve stem connector 102 has a threaded upper cavity 124 that includes a plurality of threads 126 to threadingly engage the actuator rod 104. Similarly, the valve stem connector 102 has a threaded lower cavity 128 that includes a plurality of threads 130 to threadingly engage the valve stem 106. The actuator rod 104 and the valve stem 106 are joined when the upper threads 126 and the lower threads 130 mechanically engage corresponding external threads 132 and 134 (FIG. 1) on the actuator rod 104 and the valve stem 106, respectively. In this example, because the actuator rod 104 and the valve stem 106 have different diameters, the upper cavity 124 and the lower cavity 128 are connected via a tapered internal surface 136.
To complete the valve assembly 100, the actuator rod 104 and the valve stem 106 are threaded in counter-rotating directions into the cavities 124 and 128, respectively, until the desired length is achieved so that the valve stem 106 and the plug 110 properly interact with the valve seat 112. Axial adjustment of the actuator rod 104 and the valve stem 106 is typically limited by the minimum number of threads that must be engaged to create a secure and safe mechanical connection. Finally, the bolts 122a-b may be tightened to create additional compressive load between the threads 126, 130, 132, and 134 to securably fasten the actuator rod 104 to the valve stem rod 106.
As mentioned above, in a pneumatically controlled valve assembly, a diaphragm (not shown) is coupled to the actuator rod 104, the position of which is controlled by the pressure on one side of the diaphragm and one or more springs on the opposite side of the diaphragm. By controlling the pressure in the space above the diaphragm, a direct-acting actuator is created. By controlling the pressure below the diaphragm, a reverse-acting actuator 104 is created. Movement of the diaphragm and, thus, the actuator rod 104 causes the valve stem 106 to open and close the valve plug 110 relative to the valve seat 112 to control the fluid flow through the valve body 120.